Nonlinear Force-free Modeling of a Long-lasting Coronal Sigmoid

Details Nonlinear Force-free Modeling of a Long-lasting Coronal Sigmoid Nonlinear Force-free Modeling of a Long-lasting Coronal Sigmoid

Oct 2009

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009apj...703.1766s&link_type=abstract

The Astrophysical Journal, Volume 703, Issue 2, pp. 1766-1777 (2009).

Astronomy and Astrophysics

Astronomy

13

Sun: Corona, Sun: Magnetic Fields, Sun: X-Rays, Gamma Rays

Scientific paper

A study of the magnetic configuration and evolution of a long-lasting quiescent coronal sigmoid is presented. The sigmoid was observed by Hinode/XRT and Transition Region and Coronal Explorer (TRACE) between 2007 February 6 and 12 when it finally erupted. We construct nonlinear force-free field models for several observations during this period, using the flux-rope insertion method. The high spatial and temporal resolution of the X-Ray Telescope (XRT) allows us to finely select best-fit models that match the observations. The modeling shows that a highly sheared field, consisting of a weakly twisted flux rope embedded in a potential field, very well describes the structure of the X-ray sigmoid. The flux rope reaches a stable equilibrium, but its axial flux is close to the stability limit of about 5 × 1020 Mx. The relative magnetic helicity increases with time from February 8 until just prior to the eruption on February 12. We study the spatial distribution of the torsion parameter α in the vicinity of the flux rope, and find that it has a hollow-core distribution, i.e., electric currents are concentrated in a current layer at the boundary between the flux rope and its surroundings. The current layer is located near the bald patch separatrix surface (BPSS) of the magnetic configuration, and the X-ray emission appears to come from this current layer/BPSS, consistent with the Titov and Démoulin model. We find that the twist angle Φ of the magnetic field increases with time to about 2π just prior to the eruption, but never reaches the value necessary for the kink instability.

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